1. Field of the Invention
This invention concerns a method and a device for bending glass sheets by gravity on a bending frame or skeleton.
2. Discussion of the Background
From Publication EP-A-0 448 447 a method and a device are known for bending glass sheets by gravity in at least two successive steps. In a first step, the glass sheets are bent by gravity to a first shape roughly corresponding to the definitive shape, then in a second step the glass sheets are bent by gravity to the definitive shape.
For carrying out this method, a frame termed a rough frame is used for the first step and a frame termed a definitive frame is used for the second step. The definitive frame, by which the definitive bending is performed in the second bending step, may be an autonomous closed ring, which is disposed inside the rough frame or outside it, at a small distance from it. In a variant, the definitive bending frame may be formed of parts of the rough frame and of movable supplementary parts. In this way it is possible to associate with the rough frame symmetrical lateral parts which fold upwards for the operation of the definitive or final bend, thus forming a definitive frame at the most highly curved lateral parts, while the median part is common with the median part of the rough frame.
This known method is suitable, notably, for the bending of glass sheets of complex shape and/or glass sheets having a pronounced double curvature and/or for glass sheets that are subjected to a counter-bending in critical zones.
When, in such a method or device, the transfer of the bent glass sheet in the form of a blank onto the definitive frame does not take place in a uniform manner, that is symmetrically about the axis of symmetry in the case in which the bent glass sheets must have such a symmetry, defects of curvature can be produced relative to the desired curvatures, which can harm the optical quality of the glass sheets.
A non-uniform transfer between the frames can take place when the two extreme lateral parts of the glass sheet are not taken over at the same instant by the definitive frame. In this case the forces acting on the lateral parts of the softened glass sheet are not identical and simultaneous. This results in an undesirable asymmetrical bending of the glass sheet which can harm its optical property.